The use of such control flaps to influence the flow characteristics in the inlet passage of an internal combustion engine is known. In the operative position swirling or turbulence is produced in the flow of the air/fuel mixture, which propagates through the inlet valves into the combustion chamber, where it leads to more efficient and more stable combustion, since through the swirling the atomisation of the fuel is improved and the speed of propagation of the flame front during the combustion process is increased. A particular result produced thereby is more stable combustion at lean air/fuel ratios. By the use of such a control flap valve, the proportion of fuel in a lean-burn engine mixture can be reduced without resulting in combustion instabilities such as, for example, misfires. This is desirable from the point of view of lowering the crude nitrogen oxide emissions, since generally leaner engine operations yield lower crude nitrogen oxide emissions.
However, in its operative position the control flap has a relatively high flow resistance. Consequently flow losses occur, restricting the maximum air/fuel throughput into the cylinders. Rather than using devices which are fixedly installed in the inlet passage for the production of turbulence, it is known to use control flaps which are variable and at higher engine speed/torque demands can be moved from the operative into the starting position at the instigation of the engine control unit in order to produce unimpeded air throughput and thus maximum volumetric efficiency of the internal combustion engine. A further effect of the reduced swirling in the starting position is to influence the spark duration and thereby affect the combustion noise.
In designing the shape of the control flaps, it is sought on the one hand to obtain the best possible swirl in the operative position, while on the other hand in the starting position the flow should be impeded as little as possible, since any additional resistance to flow reduces the available performance of the engine at full load.
Accordingly, it would be desirable to provide an inlet arrangement of the kind referred to above having control flaps such that, when they are in use, optimum swirling is produced in the operative position and minimum effect on the flow in the starting position.